A number of modulation techniques have been developed for facilitating communications in a network where multiple users are present. Such techniques include code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA). CDMA is a spread-spectrum technology that uses pseudo-random number sequences to modulate incoming data, multiple transmitters transmitting on the same signal, and orthogonal codes (Walsh codes) to correlate different communication channels. TDMA uses time slots to coordinate multiple uplink transmitters that are transmitting in the same sub-slots. Users transmit in rapid succession, one after the other, each using his/her own time slot, allowing multiple stations to share the same transmission medium (e.g., radio frequency channel) while using only a portion of the total available bandwidth. FDMA allocates different users with different carrier frequencies of the radio spectrum.
In addition to modulation techniques, protocols exist for determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). CSMA/CD (Carrier Sense Multiple Access/Collision Detection) is used by Ethernet networks to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.
Many spread spectrum modulation techniques have difficulty in high doppler environments. Doppler effects on the signal may cause phase problems with the transmission. These problems can result in the loss of communication signals.